P!nk is no stranger to the top of the charts. She first hit No. 1 in 2001 with her collaboration “Lady Marmalade” which featured Christina Aguilera, Lil’ Kim and Mya, according to Billboard. Ruess, on the other hand, reached No. 1 for the second time with “Just Give Me a Reason.” Last year, fun. hit No. 1 with their breakthrough single “We Are Young” featuring Janelle Monae.

P!nk and Ruess’ collaboration was a surprising match-up when it first premiered in February, which got us thinking: what’s the deal with male/female pop duet hits?

Of course you have your classic partners like Marvin Gaye and Tammi Terrell, but what about those unexpected pairings? The ones that cross genres or even generations, combining their star power to create one great song. In hindsight, some have fared better than others; sometimes, two hot stars pair up but only one of them has the longevity, leaving listeners scratching their heads decades later.

Here’s a look at five unexpected duets that soared to the top of the charts.